1. Field of the Invention
The present invention relates to current measurement. More particularly, the invention pertains to a phase modulator and a method for current measurement with a fiber optic in-line Sagnac interferometer.
2. Description of the Prior Art
Optical current measuring devices are increasingly expected to supersede conventional current measuring devices in the future. A known variation of an optical current measuring device provides a physically and technologically elegant solution is based on in-line Sagnac interferometer technology.
A current sensor of this type measures the electric current flowing through a current conductor by the Faraday effect. Such effect relates to rotation of the light oscillation plane of the propagating linearly polarized light parallel to the field lines of a magnetic field. By employing this effect, it is possible to produce a phase shift between two orthogonal polarization modes of a light beam passing through the in-line Sagnac interferometer provided the polarization modes are propagated in an optical measuring coil through which the current conductor passes. The phase shift between the polarization modes that is caused by the electric current is thus a measure of the current intensity. This can be measured by evaluating the pattern produced by interference of the two polarization modes that have been shifted with respect to one another.
A representative current sensor based on the in-line Sagnac interferometer, as described in the article of Blake, Tantaswadi and Carvalho, “In-Line Sagnac-Interferometer For Magnetic Field Sensing”, Proc. SPIE, vol. 2360 (1994) at pages 419 through 422, and its method of operation are explained below.
FIG. 2 is a schematic block diagram of a current sensor 1 based on an in-line Sagnac interferometer in accordance with the prior art. The current sensor 1 employing an in-line Sagnac interferometer includes a light source 2, a coupler 3, a polarizer 4, a 45 degree splice 5, a birefringence phase modulator 6, a quarter wavelength phase shifter 7, an isotropic fiber coil 8 (current measuring coil), a reflector 9, a current conductor 10, a detector 11, an amplification/evaluation unit 12 and a phase modulator driving unit 13. Light received from the light source 2 via the coupler 3 is polarized by the polarizer 4 and coupled in by means of the splice 5 at 45 degrees with respect to the polarization axes of a polarization-maintaining, birefringent fiber. As a result, two different polarization modes of approximately the same amplitude are guided in parallel through the same fiber. The splitting, customary in a conventional Sagnac interferometer, of a light beam that passes through the interferometer into two component light beams traveling in opposite directions is thus replaced, in an in-line Sagnac interferometer, by “decomposition” of the light beam into two parallel-guided polarization modes (i.e., parallel-guided “component light beams” of a primary light beam with a defined polarization). Each mode, due to the polarization-maintaining fiber, effectively uses a different optical light path. Both polarization modes pass through the birefringent phase modulator 6, preferably a Pockels cell, to produce a non-reciprocal phase shift between the polarization modes, the quarter wavelength phase shifter 7 for converting linearly polarized light into circularly polarized light (for maximum utilization of the Faraday effect) and the isotropic fiber coil 8, through which a current conductor 10 passes, to the reflector 9, which in the event of reflection, rotates the polarization of the impinging polarization modes through 90 degrees and, thus, interchanges the polarizations of the two polarization modes. Should a current flow in the conductor 10, a phase shift will be produced between the two circularly polarized polarization modes as a result of the Faraday effect. Both polarization modes return along the same light path but with different polarizations. The returned polarization modes are combined at the splice 5 and fed, via the polarizer 4 and the coupler 3, to a detector 11 as an interference light beam.
A significant advantage of a conventional Sagnac interferometer modified to form an in-line Sagnac interferometer lies in strictly reciprocal behavior. That is, without a current flow in the conductor 10, the two polarization modes, when coupled out of the polarization-maintaining fiber of the fiber coil 8, experience the same phase shift with respect to one another as they had when coupled into the polarization-maintaining fiber. Phase shifts between the two polarizations that occur as a result of spatial movements of the interferometer, temperature gradients or mechanical stresses in the fiber material, are reciprocal, compensating for one another, due to the parallel guidance of the polarization modes. The only significant effect that causes a non-reciprocal phase shift between the orthogonal polarization modes passing through the interferometer is the Faraday effect.
As mentioned, the birefringence phase modulator 6 enables the imposition of a non-reciprocal phase shift between the polarization modes for shifting an interferometer characteristic curve into a region of very high linearity. The phase modulator 6 includes a polarization-maintaining action (i.e. it does not act in a polarizing manner on the polarization modes). As a consequence, both polarization modes are guided equally but modulated differently in the birefringence phase modulator 6 with use being made of the Pockels effect.
To achieve this, the waveguide in the phase modulator 6 comprises an anisotropic electro-optical crystal so that the Pockels effect becomes direction-dependent and capable of representation as a tensor. The different effects of the electromagnetic field generated by the birefringent phase modulator 6 on the different polarizations are expressed mathematically by the fact that their influences can be described by different tensor elements. The effective refractive index for different polarization modes is changed differently by the electromagnetic field, resulting in a phase shift between the polarization modes.
A birefringence phase modulator with the properties mentioned above can be realized, for example, by a polarization-maintaining fiber wound onto a piezoceramic. A modulation signal is applied to the birefringence phase modulator by the phase modulator driving unit 13. The phase shift between the polarization modes is measured by the detector 11, whose output signal is fed to an amplification/evaluation unit 12.
A further example of a current sensor based on an in-line Sagnac interferometer is described in Frosio and Dänndliker “Reciprocal Reflection Interferometer For a Fiber-Optic Faraday Current Sensor”, Applied Optics 1, Vol. 33, No. 25 (September 1994). The in-line Sagnac interferometer described therein is quite similar to the design described above. Disturbances due to birefringence in the polarization-maintaining fiber are avoided by means of a light source that emits short-coherent light.
Another current sensor is described in published patent application DE 198 08 517 A1. Such current sensor is based on a conventional Sagnac interferometer. The essential difference from the prior example is that the phase shift is produced between two component light beams traveling in opposite directions that have been generated by beam splitting. As a result, the fiber coil (current measuring coil) is not isotropic, making the current sensor susceptible to disturbance effects due to rotation.
Patent application DE 100 44 197.1 describes a voltage sensor based on an in-line Sagnac interferometer having a design similar to the preceding current sensor. Again, a phase shift is produced between two polarization modes. However, the phase shift is not caused by the Faraday effect, but by means of a voltage provided by an additional phase modulator.